Read the documentation - there is a "DogTag help" button on the config page I mentioned, or just type /dog to open it up. It tells you all the neat tags you can use, including an entire section on cast-related tags.
The reason it "displayed fine before" is that the old cast bar text wasn't using DogTag at all; it's since been converted to use DogTags since they're much more flexible (people were always asking for configurable cast text).
I guess this is an issue with the DogTags then. The only tag that seems to be related to spell names, [CastName], returns 'Channeling' for any channeled spell, whereas the old cast bar would show the actual spell name.